The present invention relates to a furnace for treating sludges from sewage treatment processes, industrial wastes, or wastes which are crushed, dried, burned or otherwise semi-treated as desired, by melting the waste with a high-temperature furnace bed of combustible carbonaceous substance to draw off a molten product from the bottom of the bed so that the product can be used, for example, for construction purposes as an aggregate, or for reclamation without entailing the release of heavy metals.
Since low initial cost and reduced running cost are usually essential requirements for furnaces for treating industrial wastes, it is desired that such furnaces be directly heated with a combustible carbonaceous material, for example, with coke. To assure combustion for giving a high temperature needed for melting the waste, air or like gas must then be supplied to the combustible material at a high rate. What matters is therefore how to supply the gas for combustion.
With a simple system in which air or like gas is fed to a high-temperature furnace bed of combustible carbonaceous material from below to effect combustion to cause the resulting combustion gas to flow upward, dust of the waste and ash (hereinafter referred to simply as "dust") are exposed to, and scattered upward by, a large quantity of combustion gas flowing at a high speed, with the result that the dust will be released from the furnace as entrained in the combustion gas. The exhaust gas is therefore very likely to cause secondary pollution, or clog up, damage or overheat the exhaust duct due to the deposition of molten dust.
Conversely with another system in which the gas for combustion is fed to the high-temperature furnace bed from above to cause the resulting combustion gas to flow downward, the water contained in the waste will flow down along with the combustion gas, giving a reduced temperature to the lower portion of the bed. Additionally the carbon dioxide and water resulting from the combustion of the waste, or the water initially contained in the waste also flows downward with the combustion gas, undergoing reduction reaction with the carbon component of the combustible carbonaceous material in the lower portion of the furnace bed to absorb the ambient heat and eventually reducing the temperature of the lower portion of the bed. At a reduced temperature, the molten product will block up and will not be run off from the furnace smoothly.
Presumably secondary pollution and other objections due to the exhaust gas could be prevented with use of a secondary combustion chamber or combustion promoting device, or by returning the exhaust gas into the furnace, while an additional heater, if provided for the lower portion of the bed, would be useful for keeping this portion at the desired high temperature. Such means, however, would render the furnace construction complex and result in an increase in the initial cost as well as in the running cost due to damage to the structure, thus failing to fulfill the foregoing requirements.